Separator for non-aqueous secondary battery, process for producing  same, and non-aqueous secondary battery

ABSTRACT

A first object of the present invention is to provide a separator including a polyethylene microporous membrane and a heat-resistant porous layer, and that has a sufficient shutdown function and a sufficient heat resistance, and can be formed with a reduced thickness and can overcome the problem of slidability. A first aspect of the present invention is a separator for a non-aqueous secondary battery. The separator includes a microporous membrane of primarily polyethylene, and a heat-resistant porous layer of a primarily heat-resistant polymer formed on at least one surface of the microporous membrane. (1) The microporous membrane has a Gurley number of 25 to 35 sec/100 cc·μm per unit thickness. (2) The heat-resistant porous layer contains inorganic fine particles having an average particle diameter of 0.1 to 1.0 μm. (3) The inorganic fine particles are 40% to 80% in volume with respect to a total volume of the heat-resistant polymer and the inorganic fine particles. (4) The heat-resistant porous layer has a total thickness of 3 to 12 μm.

TECHNICAL FIELD

The present invention relates to a separator for a non-aqueous secondarybattery, and specifically to a separator intended to improve safety ofnon-aqueous secondary batteries.

BACKGROUND ART

A non-aqueous secondary battery as represented by a lithium-ionsecondary battery has been pervasive as the main power supply ofportable electronic devices, for example, such as cellular phones andlaptop computers. The lithium-ion secondary battery has been the subjectof ongoing development to obtain higher energy density, higher capacity,and higher output. This trend is expected to increase in the future. Tomeet such demands, it is of great importance to provide a technique thatensures high battery safety.

The separator for lithium-ion secondary batteries generally uses amicroporous membrane made from polyethylene or polypropylene. Theseparator has a function known as a shutdown function, intended toprovide safety for the lithium-ion secondary battery. The shutdownfunction refers to the separator's ability to abruptly increaseresistance when the battery temperature rises to a certain temperature.With the shutdown function, the separator shuts down the current flowwhen there is unexpected heat generation in the battery, preventingfurther temperature increase in the battery, and thereby avoidingfuming, fire, or explosion. The operating principle of the shutdownfunction is the closure of the pores in the separator, which occurs asthe material of the separator melts and deforms. In the case of aseparator made from polyethylene, the shutdown function comes intooperation at a temperature of approximately 140° C., near the meltingpoint of polyethylene. The shutdown temperature is approximately 165° C.for polypropylene separators. Because a relatively low shutdowntemperature is preferred from the standpoint of ensuring battery safety,polyethylene is more commonly used for the separator.

In addition to the shutdown function, a sufficient heat resistance isrequired for the separator of the lithium-ion secondary battery. This isfor the following reason. In conventional separators solely made frompolyethylene or other microporous membranes, the separator continues tomelt (known as “meltdown”) as the battery remains exposed to theoperating temperatures of the shutdown function after the shutdown. Thisis the intrinsic characteristic of the shutdown function which operatesaccording to the foregoing principle. The meltdown creates a shortcircuit inside the battery, and generates a large amount of heat,exposing the battery to the risk of fuming, fire, and explosion. Theseparator therefore requires, in addition to the shutdown function, aheat resistance sufficient to prevent meltdown near the operatingtemperatures of the shutdown function.

In an attempt to provide both the shutdown function and the heatresistance for the separator, there have been proposed separators thatinclude a polyethylene microporous membrane coated with a porous layermade from a heat-resistant resin such as polyimide or aromatic polyamide(see, for example, Patent Documents 1 to 5). In these separators, theshutdown function comes into operation near the melting point ofpolyethylene (about 140° C.), and, because the heat-resistant porouslayer has sufficient heat resistance, meltdown does not occur even attemperatures of 200° C. and higher. However, in this type ofconventional separators, because the thickness of the polyethylenemicroporous membrane is as thick as about 20 μm in virtually allseparators, the separator thickness exceeds 20 μm when coated with theheat-resistant porous layer. A drawback of the separators of the typeprovided with the heat-resistant porous layer, then, is the thicknessthat exceeds the thickness of about 20 μm commonly adopted by theseparators currently available in the market (those solely made frompolyethylene or other microporous membranes).

The shutdown function limits the thickness of the separators of the typeincluding the heat-resistant porous layer. Specifically, because of thecorrelation between the shutdown function and the thickness of thepolyethylene microporous membrane, the shutdown function becomes reducedwhen the thickness of the polyethylene microporous membrane is reduced.Further, the shutdown function tends to be reduced when the polyethylenemicroporous membrane is coated with the heat-resistant porous layer,compared with using the polyethylene microporous membrane alone. Forthese reasons, it has been required conventionally to provide athickness of at least 20 μm for the polyethylene microporous membrane,in order to provide a sufficient shutdown function for the separator.Patent Document 3 describes as an example a polyethylene microporousmembrane having a thickness of 4 μm. However, the publication does notdisclose anything about the shutdown function. Usually, a sufficientshutdown function cannot be obtained when the thickness of thepolyethylene microporous membrane is as small as 4 μm as in thisexample.

One way to reduce the separator thickness is to reduce the thickness ofthe heat-resistant porous layer. However, when the thickness of theheat-resistant porous layer is reduced too much, the heat resistancewill be insufficient, and heat shrinkage occurs over the entireseparator in a temperature range including and above the melting pointof polyethylene. In this connection, Patent Document 4 teaches aconfiguration in which a porous layer that contains a heat resistantnitrogen-containing aromatic polymer and a ceramic powder are formed ona polyethylene microporous membrane to improve the heat resistance ofthe heat-resistant porous layer. This technique appears to successfullyreduce the thickness of the heat-resistant porous layer without failingto provide a sufficient heat resistance. However, Patent Document 4 doesnot address the heat shrinkage issue of the separator, and as such abattery using the separator of Patent Document 4 has a possibility ofheat shrinkage under high temperature.

As described above, concerning the separator including the polyethylenemicroporous membrane and the heat-resistant porous layer, no techniqueis available that can sufficiently cope with both the shutdown functionand heat resistance issues, and, at the same time, provide a way toreduce the thickness of the separator.

From the standpoint of manufacture efficiency, there is also a need fora technique to improve the slidability of the separator provided withthe heat-resistant porous layer. Specifically, battery manufactureemploys a step in which a core is drawn out of the electronic elementproduced by winding the separator and electrodes around the core.Generally, stainless steel or other metallic material, with or without athin ceramic coating, is used as the core. In winding, the separator isfirst wound around the core before the electrodes. The heat-resistantresin such as a wholly aromatic polyamide used for the heat-resistantporous layer of the separator is very adherent to the metallic materialor ceramic material. The adhesion between the core and the separatorcauses a problem when drawing out the core from the electronic element,as it may damage the electronic element being produced.

Another challenge, then, is to improve the slidability of the separatorprovided with the heat-resistant porous layer. This issue becomes evenmore problematic in separators including the heat-resistant porous layeron the both surfaces of the polyethylene microporous membrane, because,in such separators, the heat-resistant porous layer will always be incontact with the core. The problem remains also in separators includingthe heat-resistant porous layer only on one surface of the polyethylenemicroporous membrane, because it undesirably imposes limitations on theorientation of the element, requiring the core to contact the side notprovided with the heat-resistant porous layer.

The slidability issue is not addressed in the Prior Art section ofPatent Document 4 or other publications. Patent Document 5 does addressthe slidability issue. To overcome the slidability problem, PatentDocument 5 proposes a technique to form an additional spacer layer on awholly aromatic polyamide porous layer. However, the configurationforming the spacer layer is not desirable because it involves largenumbers of steps and is complex.

Patent Document 1: JP-A-2002-355938

Patent Document 2: JP-A-2005-209570

Patent Document 3: JP-A-2005-285385

Patent Document 4: JP-A-2000-030686

Patent Document 5: JP-A-2002-151044

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

Against the foregoing background, it is a first object of the presentinvention to provide a separator including a polyethylene microporousmembrane and a heat-resistant porous layer, and that has a sufficientshutdown function and sufficient heat resistance, and can be formed witha reduced thickness and can overcome the problem of slidability.Further, the first object is to provide a process for producing suchseparators, and a non-aqueous secondary battery using the separator.

In another aspect, a second object of the present invention is toprovide a separator including a polyethylene microporous membrane and aheat-resistant porous layer, and that can sufficiently cope with boththe shutdown function and heat resistance issues, and can be formed witha reduced thickness. Further, the second object is to provide anon-aqueous secondary battery using the separator.

Means for Solving the Problems

In order to achieve the foregoing first object, a first aspect of thepresent invention provides the following:

1. A separator for a non-aqueous secondary battery, including: amicroporous membrane of primarily polyethylene; and a heat-resistantporous layer of primarily at least one kind of heat-resistant polymerselected from the group consisting of a wholly aromatic polyamide, apolyimide, a polyamide-imide, a polysulfone, and a polyether sulfone,the heat-resistant porous layer being formed on at least one surface ofthe microporous membrane,

the separator characterized in that:

(1) the microporous membrane has a Gurley number of 25 to 35 sec/100cc·μm per unit thickness;

(2) the heat-resistant porous layer contains inorganic fine particleshaving an average particle diameter of 0.1 to 1.0 μm;

(3) the inorganic fine particles are 40% to 80% in volume with respectto a total volume of the heat-resistant polymer and the inorganic fineparticles; and

(4) the heat-resistant porous layer has a total thickness of 3 to 12 μmwhen formed on both surfaces of the microporous membrane, and athickness of 3 to 12 μm when formed on only one surface of themicroporous membrane.

2. The separator according to 1, wherein the heat-resistant porous layeris formed on the both surfaces of the microporous membrane.

3. The separator according to 1 or 2, wherein the heat-resistant polymeris a wholly aromatic polyamide.

4. The separator according to 3, wherein the wholly aromatic polyamideis a meta-type wholly aromatic polyamide.

5. The separator according to 1, wherein the heat-resistant porous layeris formed on the both surfaces of the microporous membrane, wherein themicroporous membrane is formed of polyethylene, and wherein theheat-resistant polymer is a meta-type wholly aromatic polyamide.

6. The separator according to 4 or 5, wherein the meta-type whollyaromatic polyamide is a poly-m-phenyleneisophthalamide.

7. The separator according to any one of 1 through 6, wherein theinorganic fine particles are made of alumina, and wherein the inorganicfine particles are 65% to 90% in weight with respect to a total weightof the heat-resistant polymer and the inorganic fine particles.

8. The separator according to 7, wherein the inorganic fine particlesare made of α-alumina.

9. The separator according to any one of 1 through 8, wherein themicroporous membrane has a penetration strength of 250 g or more.

10. The separator according to any one of 1 through 9, wherein themicroporous membrane has a thickness of 7 to 16 μm, and wherein theseparator has a thickness of 20 μm or less as a whole.

11. A producing process of a separator for a non-aqueous secondarybattery, the separator including a microporous membrane of primarilypolyethylene, and a heat-resistant porous layer of a primarily whollyaromatic polyamide formed on at least one surface of the microporousmembrane,

the process characterized by including:

(1) applying a coating liquid to at least one surface of the microporousmembrane, the coating liquid including a wholly aromatic polyamide,inorganic fine particles, a solvent for dissolving the wholly aromaticpolyamide, and a solvent that serves as a poor solvent for the whollyaromatic polyamide;

(2) solidifying the coating liquid by immersing the microporous membranein a mixture of the solvent for dissolving the wholly aromaticpolyamide, and the solvent that serves as a poor solvent for the whollyaromatic polyamide, after applying the coating liquid to the microporousmembrane;

(3) performing water washing to remove the solvent mixture; and

(4) performing drying to remove the water.

12. The process according to 11, wherein the coating liquid is a slurryin which the wholly aromatic polyamide is dissolved, and in which theinorganic fine particles are dispersed.

13. The process according to 12, wherein the wholly aromatic polyamideis a meta-type wholly aromatic polyamide.

14. A non-aqueous secondary battery using a separator of any one of 1through 10.

Further, as a result of studies on the second object, the inventor ofthe present invention found that both the shutdown function and heatresistance requirements can be satisfied even with a microporousmembrane sufficiently thinner than conventional microporous membranes,when a microporous membrane of primarily polyethylene, and aheat-resistant porous layer are combined under specific conditions. Asecond aspect of the present invention was completed upon this finding.Specifically, in order to achieve the foregoing second object, thesecond aspect of the present invention provides the following:

15. A separator for a non-aqueous secondary battery, including: amicroporous membrane of primarily polyethylene; and a heat-resistantporous layer of primarily at least one kind of heat-resistant polymerselected from the group consisting of a wholly aromatic polyamide, apolyimide, a polyamide-imide, a polysulfone, and a polyether sulfone,the heat-resistant porous layer being formed on at least one surface ofthe microporous membrane,

the separator characterized in that:

(1) the microporous membrane has a Gurley number of 25 to 35 sec/100cc·μm per unit thickness;

(2) the microporous membrane has a thickness of 7 to 16 μm;

(3) the heat-resistant polymer is coated in an amount of 2 to 3 g/m²;

(4) the heat-resistant porous layer has a total thickness of 3 to 7 μmwhen formed on both surfaces of the microporous membrane, and athickness of 3 to 7 μm when formed on only one surface of themicroporous membrane; and

(5) the heat-resistant porous layer has a porosity of 40% to 60%.

16. The separator according to 15, wherein the microporous membrane hasa heat shrinkage rate at 105° C. of 10% or less both in a MD directionand a TD direction.

17. The separator according to 15 or 16, wherein the microporousmembrane has a penetration strength of 250 g or more.

18. The separator according to any one of 15 through 17, wherein theheat-resistant porous layer is formed on the both surfaces of themicroporous membrane, wherein the microporous membrane is formed ofpolyethylene, and wherein the heat-resistant polymer is a meta-typewholly aromatic polyamide.

19. The separator according to 18, wherein the meta-type wholly aromaticpolyamide is a poly-m-phenyleneisophthalamide.

20. A non-aqueous secondary battery using a separator of any one of 15through 19.

ADVANTAGE OF THE INVENTION

A separator of the present invention improves safety of non-aqueoussecondary batteries. The separator therefore finds optimum use in ahigh-energy-density, high-capacity, or high-output-density non-aqueoussecondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph concerning an example according to the first aspect ofthe present invention, representing a relationship between heatshrinkage rate and the amount of inorganic fine particles added.

FIG. 2 is a graph concerning an example according to the first aspect ofthe present invention, representing a result of the evaluation of ashutdown function.

FIG. 3 is a graph concerning an example according to the second aspectof the present invention, representing a relationship between thecoating amount of and the heat shrinkage rate of a separator.

FIG. 4 is a graph concerning an example according to the second aspectof the present invention, representing a result of the evaluation of ashutdown function.

BEST MODE FOR CARRYING OUT INVENTION First Aspect of the Invention

The first aspect of the present invention is described below in detail.

[A Separator for a Non-Aqueous Secondary Battery]

The first aspect of the present invention is a separator for anon-aqueous secondary battery,

the separator including:

a microporous membrane of primarily polyethylene; and

a heat-resistant porous layer of primarily at least one kind ofheat-resistant polymer selected from the group consisting of a whollyaromatic polyamide, a polyimide, a polyamide-imide, a polysulfone, and apolyether sulfone, the heat-resistant porous layer being formed on atleast one surface of the microporous membrane,

the separator characterized in that:

(1) the microporous membrane has a Gurley number of 25 to 35 sec/100cc·μm per unit thickness;

(2) the heat-resistant porous layer contains inorganic fine particleshaving an average particle diameter of 0.1 to 1.0 μm;

(3) the inorganic fine particles are 40% to 80% in volume with respectto a total volume of the heat-resistant polymer and the inorganic fineparticles; and

(4) the heat-resistant porous layer has a total thickness of 3 to 12 μmwhen formed on both surfaces of the microporous membrane, and athickness of 3 to 12 μm when formed on only one surface of themicroporous membrane.

By satisfying the foregoing conditions (1) to (4) at the same time, theseparator can exhibit desirable shutdown characteristics, effectivelyprevent heat shrinkage at high temperatures, and can be formed with areduced thickness. It is also possible to overcome the slidabilityproblem of the heat-resistant porous layer.

A separator for a non-aqueous secondary battery according to the firstaspect of the present invention preferably has a thickness about thesame as or less than the thicknesses of conventional separators. Athickness of 20 μm or less is preferable to meet the demands for a thinseparator.

Preferably, the separator, as a whole, has a Gurley number of 200 to1,000 sec/100 cc, more preferably 250 to 500 sec/100 cc. A Gurley numberless than 200 sec/100 cc causes the problem of insufficient heatresistance in the separator. A Gurley number exceeding 1,000 sec/100 ccabruptly increases the resistance of the separator, and the cyclecharacteristics and discharge characteristics of the battery using theseparator become insufficient.

The microporous membrane of primarily polyethylene used in a separatoraccording to the first aspect of the present invention includes thereinlarge numbers of micropores, which are in communication with one anotherso as to allow for passage of a gas or a liquid from one surface to theother of the microporous membrane. Further, the microporous membrane isadapted to exhibit the shutdown function by closing the micropores uponheating to a predetermined temperature.

The thickness of the microporous membrane is about the same as or lessthan the thicknesses of microporous membranes used in conventionalseparators. A thin microporous membrane is preferred in the batteryseparator. However, in a configuration of the present invention, amicroporous membrane thickness less than 7 μm is not preferable becauseit may lead to insufficient strength or insufficient shutdown function.Because of many industrially useful advantages offered by thinseparators, there is an underlying demand for thin separators. To obtaina thin separator, the thickness of the microporous membrane shouldpreferably be 7 to 16 μm.

Generally, forming the heat-resistant porous layer on the both surfacesor one surface of the microporous membrane of primarily polyethyleneprevents meltdown, but it considerably lowers the performance of theshutdown function. Because the site of the shutdown function is themicroporous membrane, reducing the thickness of the microporous membraneseriously impairs the shutdown function. However, with a Gurley numberof 25 to 35 sec/100 cc·μm per unit thickness of the microporous membraneas in a separator according to the first aspect of the presentinvention, a desirable shutdown function can be obtained, and the ionpermeability, involved in battery performance, becomes desirable evenwhen the thickness of the microporous membrane is as thin as about 7 to16 μm. A Gurley number per unit thickness of the microporous membraneless than 25 sec/100 cc·μm is not practical because it considerablylowers the performance of the shutdown function. When the Gurley numberper unit thickness of the microporous membrane is in excess of 35sec/100 cc·μm, the ion permeability becomes insufficient, and theresistance of the separator increases.

There is a correlation between the Gurley number of the microporousmembrane of primarily polyethylene, and porosity. Accordingly, themicroporous membrane in the first aspect of the present invention shouldappropriately have a porosity of 20% to 40%. A microporous membraneporosity less than 20% may lead to insufficient ion permeability. Whenthe porosity exceeds 40%, the Gurley number becomes small, and this maylead to insufficient shutdown characteristics.

Preferably, the microporous membrane has a penetration strength of atleast 250 g. When the penetration strength is less than 250 g, theessential function of the separator to prevent shorting in the batterywill be lost, making it difficult to ensure safety.

In the first aspect of the present invention, the microporous membraneof primarily polyethylene may include polyolefins such as polypropyleneand polymethylpentene, in addition to polyethylene. In terms ofstrength, a configuration including polypropylene is more preferablewhen using a low-molecular-weight polyethylene. Further, the microporousmembrane may include, for example, a layer of polyethylene laminated toa layer of polypropylene. In this case, the polypropylene layer ispreferably adapted to include a top layer that contains at least 80weight % polyethylene, because the polypropylene layer has poor adhesionwith the heat-resistant porous layer. When the top layer is a layer of20 weight % or more polypropylene, the adhesion with the heat-resistantporous layer can be improved by a surfactant process or a coronadischarge process.

In the first aspect of the present invention, the polyethylene used forthe microporous membrane is not particularly limited. Preferably, ahigh-density polyethylene, or a mixture of a high-density polyethyleneand an ultrahigh molecular weight polyethylene is used. Further,appropriate amounts of a low-molecular-weight polyolefin wax or anoligomer may be added. Preferably the polyethylene has a weight averagemolecular weight of 100,000 to 10,000,000. A polyethylene molecularweight of less than 100,000 and in excess of 10,000,000 is notpreferable, because, when less than 100,000, the strength of themicroporous membrane will be insufficient, and when in excess of10,000,000, molding of the membrane'becomes difficult. The weightaverage molecular weight of the polyethylene can be measured by gelpermeation chromatography (GPC). Specifically, polyethylene is dissolvedin o-dichlorobenzene at 140° C., and the solution so obtained issubjected to GPC after filtration with a sintered filter having a porediameter of 0.45 μm.

The heat-resistant porous layer of a primarily heat-resistant polymer ina separator according to the first aspect of the present inventionincludes therein large numbers of micropores, which are in communicationwith one another to allow for passage of a gas or a liquid from onesurface to the other of the heat-resistant porous layer. Theheat-resistant porous layer so structured has a heat resistancesufficient enough to prevent meltdown even in the vicinity of theshutdown temperature of the microporous membrane of primarilypolyethylene.

Preferably, the heat-resistant polymer is at least one kind selectedfrom the group consisting of a wholly aromatic polyamide, a polyimide, apolyamide-imide, a polysulfone, and a polyether sulfone. With suchheat-resistant polymers, the heat-resistant porous layer can have asufficient heat resistance. The wholly aromatic polyamide isparticularly preferable. The wholly aromatic polyamide can dissolve in apolar organic solvent, as represented by an amide solvent, inappropriate concentrations. Thus, by solidifying, water washing, anddrying a solution (coating liquid) prepared by dissolving the whollyaromatic polyamide in an organic solvent and applied over themicroporous membrane of primarily polyethylene, the heat-resistantporous layer can easily be formed into a coating. Further, control ofthe porous structure can easily be performed.

The wholly aromatic polyamide includes a meta-type and a para-type.Either form is usable in the present invention. However, meta-typewholly aromatic polyamides are preferable, andpoly-m-phenyleneisophthalamides are more preferable. The para-typewholly aromatic polyamide does not usually dissolve in an organicsolvent when it has a molecular weight necessary for usual forming, anda salt such as calcium chloride must be dissolved, or the molecularweight must be reduced to make it soluble in an organic solvent. Themeta-type wholly aromatic polyamide is more preferable over thepara-type wholly aromatic polyamide because it can be handled with lessrestriction. Further, the meta-type wholly aromatic polyamide is alsopreferable in terms of durability, because it is more resistant tooxidation and reduction than the para-type wholly aromatic polyamide.Further, because the meta-type wholly aromatic polyamide more easilyforms a porous structure than the para-type wholly aromatic polyamide, aseparator with superior permeability can be produced with goodefficiency. For example, Patent Document 4 discloses an example in whicha para-type wholly aromatic polyamide is used for the heat-resistantporous layer. Since the para-type wholly aromatic polyamide requiresaddition of ceramics for the control of a porous structure, even aporous structure cannot be obtained without adding ceramic particles.The meta-type wholly aromatic polyamide is also preferable over thepara-type wholly aromatic polyamide in this regard, because it can forma porous structure without adding ceramic particles.

In preparing a coating liquid that contains the wholly aromaticpolyamide, simply dissolving the wholly aromatic polyamide in a solventis not always sufficient to obtain a separator having a desirableshutdown function, desirable ion permeability, and desirable heatshrinkage. In those instances, it is sometimes preferable toappropriately mix a solvent that serves as a poor solvent for meta-typewholly aromatic polyamides. However, this is difficult to achievebecause the solubility for the solvent, and the coating liquid stabilitywill be insufficient for the other heat-resistant polymers such aspolyimides and para-type aromatic polyamides.

In the present invention, it is preferable that the meta-type whollyaromatic polyamide, when dissolved in N-methyl-2-pyrrolidone, have alogarithmic viscosity of from 0.8 to 2.5 dl/g, more preferably 1.0 to2.2 dl/g, as represented by formula (1) below. Outside this range,formability suffers.

Logarithmic viscosity (unit, dl/g)=ln(T/T0)/C   (1),

where T is the flow time of a solution containing 0.5 g of aromaticpolyamide dissolved in 100 ml of

N-methyl-2-pyrrolidone, as measured with a capillary viscometer at 30°C., T0 is the flow time of N-methyl-2-pyrrolidone as measured with acapillary viscometer at 30° C., and C is the polymer concentration inthe polymer solution (g/dl).

The heat-resistant porous layer in the first aspect of the presentinvention includes inorganic fine particles having an average particlediameter of 0.1 to 1.0 μm. With the heat-resistant porous layerincluding inorganic fine particles having an average particle diameterof this range, the heat resistance of the heat-resistant porous layercan be improved, and the slidability problem can be overcome at the sametime. Further, the volume ratio of the heat-resistant polymer and theinorganic fine particles can be confined within a range of the presentinvention. It is also possible to confine the thickness of theheat-resistant porous layer within a range of the present invention.When the average particle diameter of the inorganic fine particles isless than 0.1 μm, large amounts of heat-resistant polymer will berequired to bind the inorganic fine particles. This makes it difficultto confine the volume ratio of the heat-resistant polymer and theinorganic fine particles within a range of the present invention, andthe slidability cannot be improved as effectively. On the other hand,when the average particle diameter of the inorganic fine particles is inexcess of 1.0 μm, formation of a thin heat-resistant porous layerbecomes difficult, making it difficult to confine the thickness of theheat-resistant porous layer within a range of the present invention.

In a separator for a non-aqueous secondary battery according to thefirst aspect of the present invention, the volume of the inorganic fineparticles contained is 40% to 80% with respect to the total volume ofthe heat-resistant polymer and the inorganic fine particles. Adding theinorganic fine particles in such a volume ratio improves the ionpermeability of the separator, greatly reduces the heat shrinkage ratein a temperature range above the melting point of polyethylene, andenhances the performance of the shutdown function. Further, slidabilitybecomes desirable. These effects are hardly obtained when the content ofthe inorganic fine particles is less than 40 volume percent. In fact,adding the inorganic fine particles in a proportion less than 40 volumepercent is detrimental to the heat shrinkage rate and the shutdownfunction of the separator, and the effect of improving slidability willbe small. On the other hand, an inorganic fine particle content inexcess of 80 volume percent is not preferable because it causes theinorganic fine particles to fall off, which severely impairs ease ofhandling.

In examples according to the first aspect of the present invention, themicroporous membrane of primarily polyethylene has a thickness of only11 μm. Generally, the shutdown function will be insufficient when such athin microporous membrane of primarily polyethylene is coated with theheat-resistant porous layer. Further, the shutdown function will also beinsufficient when the Gurley number per unit thickness of thepolyethylene microporous membrane is simply set within a range of thepresent invention (25 to 35 sec/100 cc·μm). However, the diligent studyby the inventor of the present invention has found, rather surprisingly,that desirable shutdown characteristics can be obtained, and thehigh-temperature heat shrinkage can be sufficiently suppressed even witha thin polyethylene microporous membrane, when the content of theinorganic fine particles in the heat-resistant porous layer is 40 to 80volume percent. Note that, in a separator that undergoes large heatshrinkage in a temperature range above the melting point ofpolyethylene, a relatively desirable shutdown function can be obtainedeven when a thin polyethylene microporous membrane is used; however, theproblem of high-temperature heat shrinkage still remains, and the safetyof the separator is insufficient.

The foregoing effects of the first aspect of the present invention canbe explained as follows. First, the improper shutdown characteristicsthat occur when the heat-resistant porous layer does not includeinorganic fine particles are considered to be due to the strong adhesionof the heat-resistant polymer to polyethylene, causing theheat-resistant porous layer to seriously inhibit the closure of thepores in the microporous membrane of primarily polyethylene. Theaddition of the inorganic fine particles to the heat-resistant porouslayer is thought to improve the shutdown characteristics because theinorganic fine particles are not adherent to the polyethylenemicroporous membrane, and therefore do not inhibit the pore closure atthe interface with the microporous membrane (the interface between themicroporous membrane and the heat-resistant porous layer). Further, theinorganic particles of the heat-resistant porous layer, with the 40 to80 volume percent content range, are thought to increase the compressionmodulus of the heat-resistant porous layer, making it possible tosuppress heat shrinkage while ensuring desirable shutdowncharacteristics. This enables the thickness of the heat-resistant porouslayer to be reduced.

The inorganic fine particles for the heat-resistant porous layer are notparticularly limited. For example, oxides of alumina, titania, silica,zirconia, or the like are preferably used. Other preferable examplesinclude carbonates, phosphates, and hydroxides. From the standpoint ofimpurity dissolution and durability, it is preferable that the inorganicfine particles have high crystallinity. Further, considering chemicalstability, electrochemical stability, and specific gravity, it ispreferable to use alumina, more preferably α-alumina.

When the inorganic fine particles are made of alumina, it is preferablethat the inorganic fine particles account for 65% to 90% in weight withrespect to the total weight of the heat-resistant polymer and theinorganic fine particles. With the content of alumina fine particles inthis range, a separator can be obtained that excels in properties suchas shutdown characteristics, heat shrinkage rate, and slidability.

In the first aspect of the present invention, when the heat-resistantporous layer is formed on the both surfaces of the microporous membrane,the total thickness of the heat-resistant porous layers is preferably 3to 12 μm. When the heat-resistant porous layer is formed on only onesurface of the microporous membrane, the thickness of the heat-resistantporous layer is preferably 3 to 12 μm. In either case, when the totalthickness of the heat-resistant porous layer is less than 3 μm, asufficient heat resistance cannot be obtained, and, in particular, theeffect of suppressing the heat shrinkage will be lost. On the otherhand, when the total thickness of the heat-resistant porous layerexceeds 12 μm, it becomes difficult to provide a separator of anappropriate thickness. As used herein, the “total thickness” of theheat-resistant porous layer, when it is formed on only one surface ofthe microporous membrane, assumes that the other surface of themicroporous membrane has no thickness (0 μm).

In the first aspect of the present invention, the heat-resistant porouslayer is formed on at least one surface of the microporous membrane ofprimarily polyethylene. However, considering ease of handling,durability, and the effectiveness of heat shrinkage prevention, theheat-resistant porous layer is preferably formed on the both surfaces.Specifically, by forming the heat-resistant porous layer on the bothsurfaces of the microporous membrane, the separator will not easilycurl. This improves handling, and the separator does not easily degradeeven when the battery is used for extended time periods. Further, theheat shrinkage of the separator under high temperature, which becomesmore prominent as the thickness of the microporous membrane is reducedas in the present invention, can be appropriately prevented with theheat-resistant porous layer formed on the both surfaces of themicroporous membrane, because it stabilizes the structure. Further,because a separator according to the first aspect of the presentinvention has desirable slidability, forming the heat-resistant porouslayer on the both surfaces of the microporous membrane does not impedemanufacture of the battery. When forming the heat-resistant porous layeron the both surfaces of the microporous membrane, it is preferable thatthe thicknesses be the same on the both surfaces. The first object ofthe present invention can also be achieved with a single-sided coating;however, the single-sided coating is not as effective as thedouble-sided coating in terms of ease of handling, durability, and heatshrinkage prevention.

[Producing Process of the Separator]

A producing process of a separator for a non-aqueous secondary batteryaccording to the first aspect of the present invention is a process forproducing a non-aqueous secondary battery separator that includes amicroporous membrane of primarily polyethylene, and a heat-resistantporous layer of a primarily wholly aromatic polyamide formed on at leastone surface of the microporous membrane,

the process characterized by including:

(1) applying a coating liquid to at least one surface of the microporousmembrane, the coating liquid including a wholly aromatic polyamide,inorganic fine particles, a solvent for dissolving the wholly aromaticpolyamide, and a solvent that serves as a poor solvent for the whollyaromatic polyamide;

(2) solidifying the coating liquid by immersing the microporous membranein a mixture of the solvent for dissolving the wholly aromaticpolyamide, and the solvent that serves as a poor solvent for the whollyaromatic polyamide, after applying the coating liquid to the microporousmembrane;

(3) performing water washing to remove the solvent mixture; and

(4) performing drying to remove the water.

The solvent for dissolving the wholly aromatic polyamide in step (1) is,for example, an amide polar solvent such as N-methylpyrrolidone,dimethylacetoamide, or dimethylformamide. Use of dimethylacetoamide isparticularly preferable because it improves the properties of theproduct separator.

One feature of a producing process of a separator of the presentinvention is that the coating liquid used in step (1) includes a solventthat serves as a poor solvent for the wholly aromatic polyamide.Inclusion of the poor solvent in the coating liquid makes it possible todesirably control the inner structure of the heat-resistant porouslayer, and the structure at the interface of the heat-resistant porouslayer and the polyethylene microporous membrane. This desirably improvesthe shutdown function, ion permeability, and heat shrinkage of theproduct separator. Examples of the poor solvent include alcohols andwater. Tripropyleneglycol is particularly preferable.

A suitable ratio of the solvent for dissolving the wholly aromaticpolyamide to the solvent that serves as a poor solvent depends oncombinations of the solvents used, and as such it is difficult to give aspecific value. However, the poor solvent is preferably added to such anextent that the wholly aromatic polyamide does not precipitate.Generally, the solvent that serves as a poor solvent is added in a rangeof 5 to 50 weight %, although the present invention is not so limited.

The content of the wholly aromatic polyamide with respect to the totalof the solvent for dissolving the wholly aromatic polyamide and thesolvent that serves as a poor solvent in the coating liquid ispreferably 4 to 9 weight % . A wholly aromatic polyamide contentexceeding 9 weight % is not preferable because it lowers the ionpermeability of the separator, and the performance of the shutdownfunction. Further, in this case, the viscosity of the coating liquid,and the density of the inorganic fine particles in the slurry become toohigh, making it difficult to obtain an appropriate coating thickness. Onthe other hand, when the content of the wholly aromatic polyamide isless than 4 weight %, the wholly aromatic polyamide cannot bind theinorganic fine particles with sufficient strength. This makes itdifficult to add a sufficient amount of inorganic fine particles.

In step (1), the method of adjusting the coating liquid is notparticularly limited, as long as the wholly aromatic polyamide issufficiently dissolved, and the inorganic fine particles aresufficiently dispersed. In one exemplary method, the wholly aromaticpolyamide is dissolved in a solvent at high concentration, and theinorganic fine particles are dispersed therein before adding theremaining solvent.

When the coating liquid is applied to the both surfaces of themicroporous membrane of primarily polyethylene in step (1), it ispreferable that necessary amounts of the coating liquid be applied tothe both surfaces of the microporous membrane by simultaneouslysupplying the coating liquid from the both sides of the microporousmembrane. In one exemplary method, the excess coating liquid is suppliedto the both surfaces of the microporous membrane, and any excess amountis scraped off by passing the membrane between two opposing Meyer barsspaced apart with a predetermined distance. In another exemplary method,the coating liquid is placed between two opposing Meyer bars separatedby a predetermined distance, and the microporous membrane is passedbetween the Meyer bars to apply necessary amounts of the coating liquidon the both surfaces of the microporous membrane.

The coating method is not particularly limited when forming the coatingliquid only on one surface of the microporous membrane of primarilypolyethylene in step (1). In one exemplary method, the microporousmembrane is anchored on a glass board, and the coating liquid is appliedto one surface of the microporous membrane using the glass board and anopposing Meyer bar spaced apart with a predetermined distance.

The solidifying liquid used in step (2) is preferably a mixture of thesolvent for dissolving the wholly aromatic polyamide, the poor solvent,and water, the solvents being the same solvents used for the coatingliquid. The weight ratio of the solvent for dissolving the whollyaromatic polyamide to the solvent that serves as a poor solvent ispreferably as in the coating liquid, and the water content of thesolidifying liquid is preferably 40 to 95 weight %. The temperature ofthe solidifying liquid is preferably 10 to 70° C.

In performing the producing process above, it is more preferable to usemeta-type wholly aromatic polyamide than para-type wholly aromaticpolyamide, from the standpoint of coating liquid stability and the ionpermeability (Gurley number) of the product separator.Poly-m-phenyleneisophthalamide is particularly preferable.

A producing method of a microporous membrane of primarily polyethyleneof the present invention is not particularly limited. In one exemplaryprocess, a gel mixture of polyethylene and liquid paraffin is extrudedthrough a die, and cooled to produce a base tape, which is thenstretched to extract the liquid paraffin.

In order to appropriately adjust the Gurley number of the microporousmembrane in the foregoing process, it is preferable to adjust thepolyethylene-to-liquid paraffin ratio, the stretch rate, thepost-stretch heat fix temperature, and the post-extraction annealtemperature. The polyethylene-to-liquid paraffin ratio must be set sothat the viscosity is sufficient to allow the gel to be extruded throughthe die. In this regard, the polyethylene content is preferably 50weight % or less in the gel. Reducing the amount of polyethylene withinthis range lowers the Gurley number. The stretch rate must be set tohave a sufficient strength, which is about 10 times or more in terms ofan in-plane stretch rate. Increasing the stretch rate within this rangelowers the Gurley number. The post-stretch heat fix and thepost-extraction annealing are also intended to suppress heat shrinkage.The heat fix or annealing needs to be performed at a temperature nogreater than the melting point of polyethylene, specifically about 135°C. or less. In order to ensure the quality of the microporous membrane,the heat fix temperature must be higher than the anneal temperature. Thetemperature difference affects the Gurley number of the microporousmembrane—the greater the temperature difference, the higher the Gurleynumber. In the present invention, the temperature difference ispreferably set to about 5° C. or more. In the present invention, theGurley number of the microporous membrane can be appropriately set byappropriately adjusting these conditions.

[Battery]

A non-aqueous secondary battery according to the first aspect of thepresent invention includes an anode, a cathode, a separator according tothe first aspect of the present invention disposed between the anode andcathode, and an electrolyte. The separator according to the first aspectof the present invention provides both the shutdown function and theheat resistance at high level, and greatly improves the safety of thenon-aqueous secondary battery. In the first aspect of the presentinvention, such an effect becomes particularly prominent in anon-aqueous secondary battery having an energy density of 500 wh/L ormore, a capacity of 2.4 Ah or more, or an output density of 1.5 kwh·L ormore. The safety issue becomes more pronouncced in a battery moduleemploying serial/parallel battery connections. The non-aqueous secondarybattery according to the first aspect of the present invention is alsoeffective in this regard.

Generally, the anode includes a collector coated with a layer of ananode active material, a binder, and a conduction enhancer. To producethe anode of this construction, the anode active material, the binder,and the conduction enhancer are added to a solvent, and kneaded toobtain a slurry, which is then coated over the collector before dryingand pressing. The anode active material, the binder, and the conductionenhancer are preferably 80 to 98 weight %, 2 to 20 weight %, and 0 to 10weight %, respectively, with respect to the total 100% weight of theanode active material, the binder, and the conduction enhancer. Examplesof the anode active material include carbon materials, silicon, and tin.The carbon materials include those obtained from easy-to-graphitizepitch, such as mesocarbon microbeads and a micro carbon fiber, used asthe precursor, and those in which materials that are difficult tographitize, such as phenolic resin, are used as the precursor. Examplesof the binder include polyvinylidene fluoride and carboxymethylcellulose. Preferable examples of the conduction enhancer include agraphite powder, acetylene black,

Ketjen black, and a vapor grown carbon fiber. Preferable examples of thecollector include a copper foil and stainless steel.

As with the anode, the cathode generally includes a collector coatedwith a layer of a cathode active material, a binder, and a conductionenhancer. To produce the cathode of this construction, the cathodeactive material, the binder, and the conduction enhancer are added to asolvent, and kneaded to obtain a slurry, which is then coated over thecollector before drying and pressing. The cathode active material, thebinder, and the conduction enhancer are preferably 80 to 98 weight %, 2to 20 weight %, and 0 to 10 weight %, respectively, with respect to thetotal 100% weight of the cathode active material, the binder, and theconduction enhancer. Examples of the cathode active material includeLiCoO₂, LiNiO₂, spinel LiMn₂O₄, and olivine LiFePO₄, and solid solutionsof these materials with dissimilar elements. These materials may be usedas a mixture. As the binder, polyvinylidene fluoride is preferably used.Preferable examples of the conduction enhancer include a graphitepowder, acetylene black, Ketjen black, and a vapor grown carbon fiber.Preferable examples of the collector include an aluminum foil andstainless steel.

The electrolyte is a non-aqueous electrolyte prepared by dissolving alithium salt in a non-aqueous solvent. Preferable examples of thelithium salt include LiPF₆, LiBF₄, and LiClO₄. Examples of thenon-aqueous solvent include propylene carbonate (PC), ethylene carbonate(EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylmethylcarbonate (EMC). The lithium salts and the non-aqueous solvents may beused either alone or in a combination of two or more. The concentrationrange of the lithium salt is preferably 0.5 to 2.0 M. Consideringdurability, it is preferable to add vinylene carbonate to theelectrolyte.

In the non-aqueous secondary battery according to the first aspect ofthe present invention, it is preferable, considering durability underhigh temperature, to dispose the heat-resistant porous layer on thecathode side, when the heat-resistant porous layer is formed only on onesurface of the microporous membrane of primarily polyethylene in theseparator. Sufficient battery properties can be obtained with theheat-resistant porous layer disposed on the anode side; however,durability under high temperature will not be as good as the arrangementin which the heat-resistant porous layer is disposed on the cathodeside.

In the non-aqueous secondary battery according to the first aspect ofthe present invention, the battery element including the cathode, theanode, and the separator is sealed in a package by being wound into acylindrical or a flat structure, or by being laminated. The package maybe of any form, including a metal casing, and a casing of an aluminumlaminated film, for example.

Second Aspect of the Present Invention

The second aspect of the present invention is described below in detail.

[Separator for Non-Aqueous Secondary Battery]

A separator for a non-aqueous secondary battery according to the secondaspect of the present invention includes a microporous membrane ofprimarily polyethylene, and a heat-resistant porous layer of primarilyat least one kind of heat-resistant polymer selected from the groupconsisting of a wholly aromatic polyamide, a polyimide, apolyamide-imide, a polysulfone, and a polyether sulfone, theheat-resistant porous layer being formed on at least one surface of themicroporous membrane,

the separator characterized in that:

(1) the microporous membrane has a Gurley number of 25 to 35 sec/100cc·μm per unit thickness;

(2) the microporous membrane has a thickness of 7 to 16 μm;

(3) the heat-resistant polymer is coated in an amount of 2 to 3 g/m²;

(4) the heat-resistant porous layer has a total thickness of 3 to 7 μmwhen formed on both surfaces of the microporous membrane, and athickness of 3 to 7 μm when formed on only one surface of themicroporous membrane; and

(5) the heat-resistant porous layer has a porosity of 40% to 60%.

By satisfying the foregoing conditions (1) to (5) at the same time, theseparator can exhibit desirable shutdown characteristics, effectivelyprevent heat shrinkage at high temperatures, and can be formed with areduced thickness. It is therefore possible to improve the safety of thenon-aqueous secondary battery.

According to the second aspect of the present invention, the propertyrequirements can be satisfied even when the thickness of the separatoris 20 μm or less. Accordingly, it is preferable that the separatoraccording to the second aspect of the present invention have a thicknessof 20 μm or less, more preferably 10 to 18 μm. When the thickness of theseparator is less than 10 μm, it becomes difficult to obtain sufficientstrength and ensure safety even with a technique of the presentinvention.

The microporous membrane of primarily polyethylene used for a separatoraccording to the second aspect of the present invention includes thereinlarge numbers of micropores, which are in communication with one anotherto allow for passage of a gas or a liquid from one surface to the otherof the microporous membrane. The microporous membrane is structured sothat, when heated to a predetermined temperature, the micropores thereinclose to exhibit the shutdown function.

In the second aspect of the present invention, the microporous membranemust have a thickness of 7 to 16 μm. In a technique of the presentinvention, when the thickness of the microporous membrane is less than 7μm, a sufficient strength and a sufficient shutdown function cannot beobtained. When in excess of 16 μm, the thickness of the separator as awhole will be no different from the thickness of conventionalseparators.

In the second aspect of the present invention, the microporous membraneof primarily polyethylene has a Gurley number of 25 to 35 sec/100 cc·μmper unit thickness, and the heat-resistant porous layer has a porosityof 40% to 60%. This makes it possible to sufficiently maintain theshutdown characteristics and prevent meltdown even with the microporousmembrane of the thickness as thin as 7 to 16 μm. A Gurley number lessthan 25 sec/100 cc·μm per unit thickness of the microporous membrane isnot practical because it considerably lowers the performance of theshutdown function upon coating the heat-resistant porous layer. When theGurley number is in excess of 35 sec/100 cc·μm, the ion permeabilitybecomes insufficient, and the resistance of the separator increasesundesirably. Further, the ion permeability will be insufficient, and theseparator resistance increases when the porosity of the heat-resistantporous layer is less than 40%. A porosity of the heat-resistant porouslayer exceeding 60% is not preferable because it may lead to significantdecrease in the performance of the shutdown function, or insufficientheat resistance.

In the second aspect of the present invention, the heat-resistantpolymer is coated in an amount of 2 to 3 g/m². When the coating amountis less than 2 g/m², it may not be possible to sufficiently prevent theheat shrinkage of the separator. On the other hand, a coating amountexceeding 3 g/m² is not preferable because, in this case, the porosityof the heat-resistant porous layer becomes too low when the thickness ofthe heat-resistant porous layer is about 7 μm. Further, an attempt toset an appropriate porosity overly increases the thickness of theheat-resistant porous layer.

In the second aspect of the present invention, when the heat-resistantporous layer is formed on the both surfaces of the microporous membrane,the total thickness of the heat-resistant porous layers is preferably 3to 7 μm. When the heat-resistant porous layer is formed on only onesurface of the microporous membrane, the thickness of the heat-resistantporous layer is preferably 3 to 7 μm. In either case, when the totalthickness of the heat-resistant porous layer is 3 μm or less, asufficient heat resistance will not be obtained, and, in particular, theeffect of suppressing the heat shrinkage will be lost. On the otherhand, when the total thickness of the heat-resistant porous layerexceeds 7 μm, it becomes difficult to provide a separator of anappropriate thickness. As used herein, the “total thickness” of theheat-resistant porous layer, when it is formed on only one surface ofthe microporous membrane, assumes that the other surface of themicroporous membrane has no thickness (0 μm).

There is a correlation between the Gurley number of the microporousmembrane of primarily polyethylene, and porosity. Accordingly, themicroporous membrane in the second aspect of the present inventionshould appropriately have a porosity of 20% to 40%. A microporousmembrane porosity less than 20% may lead to insufficient ionpermeability. When the porosity exceeds 40%, the Gurley number becomessmall, and this may lead to insufficient shutdown characteristics.

In the second aspect of the present invention, it is preferable that theheat shrinkage rate of the microporous membrane at 105° C. be 10% orless, both in MD direction and TD direction. When the heat shrinkagerate of the microporous membrane exceeds 10%, it becomes difficult tosufficiently prevent heat shrinkage even when the heat-resistant porouslayer configured according to the present invention is coated on onesurface or both surfaces of the microporous membrane.

Preferably, the microporous membrane has a penetration strength of 250 gor more. When the penetration strength is less than 250 g, the essentialfunction of the separator to prevent shorting in the battery maybe lost,making it difficult to ensure safety.

Regarding the materials usable for the microporous membrane according tothe second aspect of the present invention, no further explanation willbe made because they are as in the foregoing first aspect of the presentinvention.

The heat-resistant porous layer of a primarily heat-resistant polymer ina separator according to the second aspect of the present inventionincludes therein large numbers of micropores, which are in communicationwith one another to allow for passage of a gas or a liquid from onesurface to the other of the heat-resistant porous layer. Theheat-resistant porous layer so structured has a heat resistancesufficient enough to prevent meltdown even in the vicinity of theshutdown temperature of the microporous membrane of primarilypolyethylene.

Preferably, the heat-resistant polymer is at least one kind ofheat-resistant polymer selected from the group consisting of a whollyaromatic polyamide, a polyimide, a polyamide-imide, a polysulfone, and apolyether sulfone. The wholly aromatic polyamide is preferable,meta-type wholly aromatic polyamide is more preferable, andpoly-m-phenyleneisophthalamide is particularly preferable. The reasonsthat meta-type wholly aromatic polyamide, and particularlypoly-m-phenyleneisophthalamide are preferable, and the specificconstitution of the meta-type wholly aromatic polyamide are essentiallyas in the foregoing first aspect of the present invention, and nofurther explanation will be made.

In the second aspect of the present invention, the heat-resistant porouslayer is formed on at least one surface of the microporous membrane ofprimarily polyethylene. However, considering ease of handling,durability, and the effectiveness of heat shrinkage prevention, theheat-resistant porous layer is preferably formed on the both surfaces.The reason for this is as described in the foregoing first aspect of thepresent invention. When forming the heat-resistant porous layer on theboth surfaces of the microporous membrane, it is preferable that thethicknesses be the same on the both surfaces.

[Producing Process of the Separator]

A producing process of a separator for a non-aqueous secondary batteryaccording to the second aspect of the present invention includes thefollowing steps (1) to (4):

(1) applying a coating liquid to at least one surface of the microporousmembrane, the coating liquid including a wholly aromatic polyamide, asolvent for dissolving the wholly aromatic polyamide, and a solvent thatserves as a poor solvent for the wholly aromatic polyamide;

(2) solidifying the coating liquid by immersing the microporous membranein a mixture of the solvent for dissolving the wholly aromaticpolyamide, and the solvent that serves as a poor solvent for the whollyaromatic polyamide, after applying the coating liquid to the microporousmembrane;

(3) performing water washing to remove the solvent mixture; and

(4) performing drying to remove the water.

Note that the elements of steps (1) and (2), such as the solvent fordissolving the wholly aromatic polyamide, the solvent that serves as apoor solvent for the wholly aromatic polyamide, the mixture ratio of thesolvents, the concentration of the wholly aromatic polyamide, thecoating method, and the constitution of the solidifying liquid areessentially as in the foregoing first aspect of the present invention.Further, the producing method of the microporous membrane of primarilypolyethylene is as in the first aspect of the present invention. Assuch, no further explanation will be made regarding these.

[Battery]

A non-aqueous secondary battery according to the second aspect of thepresent invention includes an anode, a cathode, a separator according tothe second aspect of the present invention disposed between the anodeand cathode, and an electrolyte. The separator according to the secondaspect of the present invention provides both the shutdown function andthe heat resistance at high level, and greatly improves the safety ofthe non-aqueous secondary battery. In the second aspect of the presentinvention, such an effect becomes particularly prominent in anon-aqueous secondary battery having an energy density of 500 wh/L ormore, a capacity of 2.4 Ah or more, or an output density of 1.5 kwh·L ormore. The safety issue becomes more pronounced in a battery moduleemploying serial/parallel battery connections. The non-aqueous secondarybattery according to the second aspect of the present invention is alsoeffective in this regard.

Note that the specific configuration of the non-aqueous secondarybattery according to the second aspect of the present invention isessentially as in the foregoing first aspect of the present invention,and accordingly no further explanation will be made.

Examples Examples According to the First Aspect of the Present Invention

Examples according to the first aspect of the present invention aredescribed below.

Measurement Methods [Average Particle Diameter]

Measurement was made using a laser diffraction particle distributionmeasurement device. Water was used as the dispersion medium of theinorganic fine particles, and a trace amount of a nonionic surfactantTriton X-100 was used as the dispersant. The medium particle diameter(D50) of the resulting volume particle distribution was taken as theaverage particle diameter.

[Gurley Number]

Measurement of Gurley number was made according to JIS P8117.

[Thickness]

The thickness of the polyethylene microporous membrane was measured at atotal of 20 points, using a contact-type thickness meter (DigimaticIndicator: Mitsutoyo Corporation), and the average of these points wastaken. Measurement was performed under the setting that a load of 1.2kg/cm² was applied to the contact terminal when measuring the thickness.The thickness of the porous layer was measured by subtracting thethickness of the polyethylene microporous membrane from the thickness ofthe product separator measured in the same manner.

[Heat Shrinkage Rate]

The heat shrinkage rate was measured as follows. A sample measuring 18cm (MD direction)×6 cm (TD direction) was cut out. The sample was markedat two locations: point A, 2 cm from the top; and point B, 17 cm fromthe top, on the perpendicular bisector of the TD. The sample was alsomarked at two locations: point C, 1 cm from the left; and point D, 5 cmfrom the left, on the perpendicular bisector of the MD. With a clip, thesample was hung in an oven adjusted to 175° C., where it was heattreated for 30 minutes under no tension. The distance between points Aand B, and the distance between points C and D were measured before andafter the heat treatment, and the heat shrinkage rate was determinedusing the following equations.

MD heat shrinkage rate={(distance AB before heat treatment−distance ABafter heat treatment)/distance AB before heat treatment}×100

TD heat shrinkage rate={(distance CD before heat treatment−distance CDafter heat treatment)/distance CD before heat treatment}×100

[Penetration Strength]

A penetration test was performed using a KES-G5 handy compression testeravailable from Kato Tech Co., Ltd. (curvature radius at the needle tip,0.5 mm; penetration speed, 2 mm/sec). The maximum penetration load wastaken as the penetration strength. In the test, the sample was anchoredon a metal frame (sample holder) having an 11.3 mm hole (ø), by beingclamped with a silicone rubber packing.

[Slidability 1]

The separator was dragged on a SUS board. The slidability of the samplewas deemed as being “desirable” when it slid without any difficulty, and“undesirable” when it had difficulty sliding.

[Slidability 2]

The slidability of the separator was evaluated by measuring acoefficient of friction of the separator, using a card friction testeravailable from Toyo Seiki. Specifically, the separator was attached to a1-kg load weight, and the force required to push the weight with theseparator in contact with the SUS stage surface of the tester wasmeasured. The coefficient of friction was determined from the force soobtained, and a normal force. The separator was attached to a 7-cm²planar area of the weight.

Example 1-1

A polyethylene microporous membrane was used that had a unit weight of6.99 g/m², a thickness of 11 μm, a Gurley number of 322 seconds (29.3sec/100 cc·μm), and a penetration strength of 443 g. The polyethylenemicroporous membrane had a weight average molecular weight of 1,270,000.

As the meta-type wholly aromatic polyamide, thepoly-m-phenyleneisophthalamide Conex® (Teijin Techno Products Limited)was used. As the inorganic fine particles, α-alumina having an averageparticle diameter of 0.8 μm (Iwatani Kagaku Kogyo, SA-1) was used. TheConex and the alumina were adjusted to a weight ratio of 30:70 (volumeratio of 55:45), and the mixture was added to a 60:40 weight-ratio mixedsolvent of dimethylacetoamide (DMAc) and tripropyleneglycol (TPG) so asto make the Conex content 6 weight %. The resultant mixture was obtainedas the coating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. Here, the clearance betweenthe Meyer bars was adjusted to 30 μm, and the both Meyer bars were ofsize 6. The polyethylene microporous membrane was immersed in asolidifying liquid at 40° C. (water:mixed solvent=50:50, weight ratio),followed by water washing and drying. As a result, a heat-resistantporous layer was formed on the both surfaces of the polyethylenemicroporous membrane.

The properties of the separator for a non-aqueous secondary battery ofthe present invention produced as above are as follows.

Thickness: 15 μm

Thickness of coating layer: 4 μm

Coating amount: 5.26 g/m²

Gurley number: 447 sec/100 cc

Heat shrinkage rate at 175° C.: 22% in MD direction, 19% in TD direction

Slidability: Desirable

Coefficient of friction: 0.52

Example 1-2

The same polyethylene microporous membrane, meta-type wholly aromaticpolyamide, and inorganic fine particles of Example 1-1 were used.

The Conex and the alumina were adjusted to a weight ratio of 20:80(volume ratio of 42:58), and the mixture was added to a 60:40weight-ratio mixed solvent of dimethylacetoamide (DMAc) andtripropyleneglycol (TPG) so as to make the Conex content 6 weight %. Theresultant mixture was obtained as the coating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. Here, the clearance betweenthe Meyer bars was adjusted to 30 μm, and the both Meyer bars were ofsize 6. The polyethylene microporous membrane was immersed in asolidifying liquid at 40° C. (water:mixed solvent=50:50, weight ratio),followed by water washing and drying. As a result, a heat-resistantporous layer was formed on the both surfaces of the polyethylenemicroporous membrane.

The properties of the separator for a non-aqueous secondary battery ofthe present invention produced as above are as follows.

Thickness: 15 μm

Thickness of coating layer: 4 μm

Coating amount: 5.64 g/m²

Gurley number: 435 sec/100 cc

Heat shrinkage rate at 175° C.: 21% in MD direction, 18% in TD direction

Slidability: Desirable

Coefficient of friction: 0.49

Example 1-3

The same polyethylene microporous membrane and meta-type wholly aromaticpolyamide of Example 1-1 were used.

As the inorganic fine particles, α-alumina having an average particlediameter of 0.8 μm (Showa Denko, AL160 SG-3) was used. The Conex and thealumina were adjusted to a weight ratio of 15:85 (volume ratio of34:66), and the mixture was added to a 50:50 weight-ratio mixed solventof dimethylacetoamide (DMAc) and tripropyleneglycol (TPG) so as to makethe Conex content 5.5 weight %. The resultant mixture was obtained asthe coating liquid.

Two Meyer bars were disposed opposite to each other. The clearancebetween the Meyer bars was adjusted to 30 μm, and the both Meyer barswere of size 6. The polyethylene microporous membrane was passed betweenthe Meyer bars by drawing it while supplying the coating liquid from theboth sides of the Meyer bars, so as to apply the coating liquid on theboth surfaces of the polyethylene microporous membrane. Then, thepolyethylene microporous membrane was immersed in a solidifying liquidat 40° C. (water:mixed solvent=50:50, weight ratio), followed by waterwashing and drying. As a result, a porous layer of Conex and alumina wasformed on the both surfaces of the polyethylene microporous membrane.

The properties of the separator for a non-aqueous secondary battery ofthe present invention produced as above are as follows.

Thickness: 18 μm

Thickness of coating layer: 7 μm

Coating amount: 8.09 g/m²

Gurley number: 380 sec/100 cc

Heat shrinkage rate at 175° C.: 11% in MD direction, 10% in TD direction

Slidability: Desirable

Coefficient of friction: 0.45

Example 1-4

The same polyethylene microporous membrane and coating liquid of Example1-3 were used.

The polyethylene microporous membrane was anchored on a glass board, andthe coating liquid was applied on one surface of the polyethylenemicroporous membrane using a Meyer bar of size 6. The clearance betweenthe Meyer bar and the polyethylene microporous membrane was 10 Afterapplying the coating liquid, the polyethylene microporous membrane wasimmersed in a solidifying liquid at 40° C. (water:mixed solvent=50:50,weight ratio), followed by water washing and drying. As a result, aheat-resistant porous layer was formed on one surface of thepolyethylene microporous membrane.

The properties of the separator for a non-aqueous secondary battery ofthe present invention produced as above are as follows.

Thickness: 16 μm

Thickness of coating layer: 5 μm

Coating amount: 6.56 g/m²

Gurley number: 364 sec/100 cc

Heat shrinkage rate at 175° C.: 19% in MD direction, 18% in TD direction

Slidability: Desirable

Coefficient of friction: 0.44

Note that curling occurred in the single-coated separator of Example1-4, and the ease of handling was not as preferable as that of thedouble-coated separator of Example 1-3.

Example 1-5

The same polyethylene microporous membrane of Example 1-1 was used.

151 parts by weight of calcium chloride was added to 2,200 parts byweight of N-2-methylpyrrolidone, and the mixture was heated to 100° C.to completely dissolve the calcium chloride. The solution was allowed tocool to room temperature, and 68.23 parts by weight of para-phenylenediamine was added and completely dissolved. Then, 124.97 g ofterephthalic acid dichloride was added to the solution at 20° C. Themixture was aged for one hour at a maintained temperature of 20° C. withstirring, and filtered with a 1,500-mesh stainless steel wire mesh. Byadding tripropyleneglycol (TPG), a solution was obtained that contained6 weight % p-phenylene terephthalamide dissolved in a 95:5 weight-ratiomixture of N-2-methylpyrrolidone and tripropyleneglycol (TPG).

Then, α-alumina having an average particle diameter of 0.8 μm (ShowaDenko, AL160 SG-3) was added to the solution and dispersed therein at ap-phenylene terephthalamide:alumina weight ratio of 20:80 (volumeratio=42:58), so as to obtain a coating liquid.

Two Meyer bars were disposed opposite to each other. The clearancebetween the Meyer bars was adjusted to 30 μm, and the both Meyer barswere of size 6. The polyethylene microporous membrane was passed betweenthe Meyer bars by drawing it while supplying the coating liquid from theboth sides of the Meyer bars, so as to apply the coating liquid on theboth surfaces of the polyethylene microporous membrane. Then, thepolyethylene microporous membrane was immersed in a solidifying liquidat 40° C. (water:N-2-methylpyrrolidone=50:50, weight ratio), followed bywater washing and drying. As a result, a heat-resistant porous layer wasformed on the both surfaces of the polyethylene microporous membrane.

The properties of the separator for a non-aqueous secondary battery ofthe present invention produced as above are as follows.

Thickness: 18 μm

Thickness of coating layer: 7 μm

Coating amount: 7.23 g/m²

Gurley number: 936 sec/100 cc

Heat shrinkage rate at 175° C.: 14% in MD direction, 13% in TD direction

Slidability: Desirable

Coefficient of friction: 0.51

Comparative Example 1-1

The same polyethylene microporous membrane, meta-type wholly aromaticpolyamide, and inorganic fine particles of Example 1-1 were used.

The Conex and the alumina were adjusted to a weight ratio of 90:10(volume ratio of 96:4), and the mixture was added to a 60:40weight-ratio mixed solvent of dimethylacetoamide (DMAc) andtripropyleneglycol (TPG) so as to make the Conex content 6 weight %. Theresultant mixture was obtained as the coating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. Here, the clearance betweenthe Meyer bars was adjusted to 30 μm, and the both Meyer bars were ofsize 6. The polyethylene microporous membrane was immersed in asolidifying liquid at 40° C. (water:mixed solvent=50:50, weight ratio),followed by water washing and drying. As a result, a heat-resistantporous layer was formed on the both surfaces of the polyethylenemicroporous membrane.

The properties of the separator for a non-aqueous secondary batteryproduced as above are as follows.

Thickness: 15 μm

Thickness of coating layer: 4 μm

Coating amount: 2.38 g/m²

Gurley number: 453 sec/100 cc

Heat shrinkage rate at 175° C.: 37% in MD direction, 23% in TD direction

Slidability: Undesirable

Coefficient of friction: 0.95

Comparative Example 1-2

The same polyethylene microporous membrane, meta-type wholly aromaticpolyamide, and inorganic fine particles of Example 1-1 were used.

The Conex and the alumina were adjusted to a weight ratio of 50:50(volume ratio of 74:26), and the mixture was added to a 60:40weight-ratio mixed solvent of dimethylacetoamide (DMAc) andtripropyleneglycol (TPG) so as to make the Conex content 6 weight %. Theresultant mixture was obtained as the coating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. Here, the clearance betweenthe Meyer bars was adjusted to 30 μm, and the both Meyer bars were ofsize 6. The polyethylene microporous membrane was immersed in asolidifying liquid at 40° C. (water:mixed solvent=50:50, weight ratio),followed by water washing and drying. As a result, a heat-resistantporous layer was formed on the both surfaces of the polyethylenemicroporous membrane.

The properties of the separator for a non-aqueous secondary batteryproduced as above are as follows.

Thickness: 15 μm

Thickness of coating layer: 4 μm

Coating amount: 2.15 g/m²

Gurley number: 452 sec/100 cc

Heat shrinkage rate at 175° C.: 53% in MD direction, 36% in TD direction

Slidability: Undesirable

Coefficient of friction: 0.75

Comparative Example 1-3

The same polyethylene microporous membrane, meta-type wholly aromaticpolyamide, and inorganic fine particles of Example 1-1 were used.

The Conex and the alumina were adjusted to a weight ratio of 5:95(volume ratio of 13:87), and the mixture was added to a 60:40weight-ratio mixed solvent of dimethylacetoamide (DMAc) andtripropyleneglycol (TPG) so as to make the Conex content 6 weight %. Theresultant mixture was obtained as the coating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. Here, the clearance betweenthe Meyer bars was adjusted to 30 μm, and the both Meyer bars were ofsize 6. The polyethylene microporous membrane was immersed in asolidifying liquid at 40° C. (water:mixed solvent=50:50, weight ratio),followed by water washing and drying. As a result, the coating layer wasdetached, and no heat-resistant porous layer was formed on thepolyethylene microporous membrane.

Comparative Example 1-4

The same polyethylene microporous membrane and meta-type wholly aromaticpolyamide of Example 1-1 were used.

As the inorganic fine particles, α-alumina having an average particlediameter of 2.0 μm (Iwatani Kagaku Kogyo, RA-1) was used. The Conex andthe alumina were adjusted to a weight ratio of 30:70 (volume ratio of55:45), and the mixture was added to a 60:40 weight-ratio mixed solventof dimethylacetoamide (DMAc) and tripropyleneglycol (TPG) so as to makethe Conex content 6 weight %. The resultant mixture was obtained as thecoating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. Here, the clearance betweenthe Meyer bars was adjusted to 50 μm, and the both Meyer bars were ofsize 8. During this process, large numbers of streaks were formed, andit was difficult to uniformly apply the coating liquid.

Comparative Example 1-5

A polyethylene microporous membrane was used that had a unit weight of7.72 g/m², a thickness of 12 a Gurley number of 257 seconds (21.4sec/100 cc˜μm), and a penetration strength of 300 g. The polyethylenemicroporous membrane had a weight average molecular weight of 530,000.

The same meta-type wholly aromatic polyamide and inorganic fineparticles of Example 1-1 were used. The Conex and the alumina wereadjusted to a weight ratio of 30:70 (volume ratio of 55:45), and themixture was added to a 60:40 weight-ratio mixed solvent ofdimethylacetoamide (DMAc) and tripropyleneglycol (TPG) so as to make theConex content 6 weight %. The resultant mixture was obtained as thecoating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. Here, the clearance betweenthe Meyer bars was adjusted to 30 μm, and the both Meyer bars were ofsize 6. The polyethylene microporous membrane was immersed in asolidifying liquid at 40° C. (water:mixed solvent=50:50, weight ratio),followed by water washing and drying. As a result, a Conex porous layerwas formed on the both surfaces of the polyethylene microporousmembrane.

The properties of the separator for a non-aqueous secondary batteryproduced as above are as follows.

Thickness: 15 μm

Thickness of coating layer: 4 μm

Coating amount: 5.07 g/m²

Gurley number: 412 sec/100 cc

Heat shrinkage rate at 175° C.: 15% in MD direction, 10% in TD direction

Slidability: Desirable

Coefficient of friction: 0.53

Comparative Example 1-6

In Comparative Example 1-6, no inorganic fine particles were added.

The same polyethylene microporous membrane and meta-type wholly aromaticpolyamide of Example 1-1 were used. The Conex was added to a 70:30weight-ratio mixed solvent of dimethylacetoamide (DMAc) andtripropyleneglycol (TPG) so as to make the Conex content 6 weight %. Theresultant mixture was obtained as the coating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. Here, the clearance betweenthe Meyer bars was adjusted to 30 μm, and the both Meyer bars were ofsize 6. The polyethylene microporous membrane was immersed in asolidifying liquid at 40° C. (water:mixed solvent=80:20, weight ratio),followed by water washing and drying. As a result, a heat-resistantporous layer was formed on the both surfaces of the polyethylenemicroporous membrane.

The properties of the separator for a non-aqueous secondary batteryproduced as above are as follows.

Thickness: 15 μm

Thickness of coating layer: 4 μm

Coating amount: 2.07 g/m²

Gurley number: 482 sec/100 cc

Heat shrinkage rate at 175° C.: 37% in MD direction, 20% in TD direction

Slidability: Undesirable

Coefficient of friction: 0.90

Comparative Example 1-7

The same polyethylene microporous membrane, meta-type wholly aromaticpolyamide, and inorganic fine particles of Example 1-3 were used.

The Conex and the alumina were adjusted to a weight ratio of 40:60(volume ratio of 66:34), and the mixture was added to a 50:50weight-ratio mixed solvent of dimethylacetoamide (DMAc) andtripropyleneglycol (TPG) so as to make the Conex content 5.5 weight %.The resultant mixture was obtained as the coating liquid.

The polyethylene microporous membrane was anchored on a glass board, andthe coating liquid was applied on one surface of the polyethylenemicroporous membrane using a Meyer bar of size 6. The clearance betweenthe Meyer bar and the polyethylene microporous membrane was 10 Afterapplying the coating liquid, the polyethylene microporous membrane wasimmersed in a solidifying liquid at 40° C. (water:mixed solvent=50:50,weight ratio), followed by water washing and drying. As a result, aheat-resistant porous layer was formed on one surface of thepolyethylene microporous membrane.

The properties of the separator for a non-aqueous secondary batteryproduced as above are as follows.

Thickness: 16 μm

Thickness of coating layer: 5 μm

Coating amount: 4.57 g/m²

Gurley number: 372 sec/100 cc

Heat shrinkage rate at 175° C.: 41% in MD direction, 30% in TD direction

Slidability: Desirable

Coefficient of friction: 0.64

It can be seen from the results of Examples 1-3, 1-4, and ComparativeExample 1-7 that similar results can be obtained regardless of whetherthe coating is formed on one surface or both surfaces of thepolyethylene microporous membrane. It should be noted here that theeffect of suppressing the heat shrinkage rate is not as effective inComparative Example 1-7, because the content of the inorganic fineparticles falls outside the range of the present invention.

Comparative Example 1-8

The same polyethylene microporous membrane and meta-type wholly aromaticpolyamide of Example 1-1 were used.

As the inorganic fine particles, α-alumina having an average particlediameter of 13 nm (Nippon Aerosil, alumina C) was used. The Conex andthe alumina were adjusted to a weight ratio of 30:70 (volume ratio of55:45), and the mixture was added to a 50:50 weight-ratio mixed solventof dimethylacetoamide (DMAc) and tripropyleneglycol (TPG) so as to makethe Conex content 5.5 weight %. The resultant mixture was obtained asthe coating liquid.

Two Meyer bars were disposed opposite to each other. The clearancebetween the Meyer bars was adjusted to 30 μm, and the both Meyer barswere of size 6. The polyethylene microporous membrane was passed betweenthe Meyer bars by drawing it while supplying the coating liquid from theboth sides of the Meyer bars, so as to apply the coating liquid on theboth surfaces of the polyethylene microporous membrane. Then, thepolyethylene microporous membrane was immersed in a solidifying liquidat 40° C. (water:mixed solvent=50:50, weight ratio), followed by waterwashing and drying. As a result, a heat-resistant porous layer wasformed on the both surfaces of the polyethylene microporous membrane.

The properties of the separator for a non-aqueous secondary batteryproduced as above are as follows.

Thickness: 17 μm

Thickness of coating layer: 6 μm

Coating amount: 6.13 g/m²

Gurley number: 395 sec/100 cc

Heat shrinkage rate at 175° C.: 21% in MD direction, 15% in TD direction

Slidability: Undesirable

Coefficient of friction: 0.88

It can be seen from the result of Comparative Example 1-8 that theslidability suffers when the average particle diameter of the inorganicfine particles is less than 0.1 μm.

Comparative Example 1-9

A porous layer of p-phenylene terephthalamide and alumina was formed onthe both surfaces of the polyethylene microporous membrane as in Example1-5, except that the weight ratio of p-phenylene terephthalamide andalumina was 70:30 (volume ratio=87:13).

The properties of the separator for a non-aqueous secondary batteryproduced as above are as follows.

Thickness: 18 μm

Thickness of coating layer: 7 μm

Coating amount: 2.53 g/m²

Gurley number: 1,568 sec/100 cc

Heat shrinkage rate at 175° C.: 48% in MD direction, 25% in TD direction

Slidability: Undesirable

Coefficient of friction: 0.86

It can be seen from the results of Example 1-5 and Comparative Example1-9 that the effect of preventing heat shrinkage can also be obtainedwith the use of p-phenylene terephthalamide, as long as the volume ratioof the wholly aromatic polyamide and the inorganic fine particles iswithin the range of the present invention. It can also be seen, however,that the p-phenylene terephthalamide, with its high Gurley number, isnot as preferable as the m-phenylene isophthalamide in terms of ionpermeability.

FIG. 1 is a graph representing the separators of Examples 1-1, 1-2, andComparative Examples 1-1, 1-2, 1-3, and 1-6 produced as above. The graphplots heat shrinkage rate at 175° C. (vertical axis) against the amountof alumina added (volume fraction of alumina with respect to the totalof meta-type wholly aromatic polyamide and alumina; horizontal axis). Itcan be seen from FIG. 1 that the alumina is detrimental to heatshrinkage when added in small amounts, but desirably suppresses the heatshrinkage rate when added in appropriate amounts.

[Evaluation of Shutdown Characteristics]

The separator produced as above was impregnated with electrolyte, andplaced between SUS boards. As the electrolyte, a 1 M solution of LiBF₄dissolved in a mixed solvent of propylene carbonate and ethylenecarbonate (1:1 weight ratio) was used. The separator so prepared wassealed in a coin cell, which was then connected to a lead, and placed inan oven with a thermocouple. The resistance was measured by applyingalternating current (amplitude, 10 mV; frequency, 1 kHz) whileincreasing the temperature at a rate of 1.6° C/min.

The results are represented in FIG. 2. By comparing the Examples and theComparative Examples, it can be seen that the separators coated with theheat-resistant porous layer can exhibit a desirable shutdown functiononly if they have a configuration of the present invention. As usedherein, the shutdown function is “desirable” when the resistancesufficiently rises within a narrow temperature range. In themeasurements of this specification, the shutdown function is deemeddesirable when the resistance reaches 10⁴ ohm·cm² within a temperaturerange of 5° C. after the resistance starts rising. Although FIG. 2 onlyrepresents results for Examples 1-1, 1-2, and Comparative Examples 1-1,1-2, 1-5, and 1-6, the shutdown characteristics were equally desirablein Examples 1-3 through 1-5 as in Examples 1-1 and 1-2. The shutdowncharacteristics were also desirable in Comparative Examples 1-7 and 1-8,but not in Comparative Example 1-9.

Table 1 summarizes the results of evaluation, along with variousconditions of the separators of Examples 1-1 through 1-5, andComparative Examples 1-1 through 1-9. It can be seen from Table 1 thatseparators with desirable shutdown characteristics (SD characteristics),heat shrinkage rate, and slidability can be obtained when the conditionsof the present invention are met.

TABLE 1 Heat-resistant porous layer Particle diameter Polyethylenemicroporous of Content of Separator, whole membrane inorganic inorganicGurley Gurley Penetration fine fine Thickness number Thickness numberstrength particles particles (μm) (sec/100 cc) (μm) (sec/100 cc · μm)(g) Polymer (μm) (volume %) Ex. 1-1 15 447 11 29.3 443 m-Aramid 0.8 45Ex. 1-2 15 435 11 29.3 443 m-Aramid 0.8 58 Ex. 1-3 18 380 11 29.3 443m-Aramid 0.8 66 Ex. 1-4 16 364 11 29.3 443 m-Aramid 0.8 66 Ex. 1-5 18936 11 29.3 443 p-Aramid 0.8 58 Com. Ex. 1-1 15 453 11 29.3 443 m-Aramid0.8  4 Com. Ex. 1-2 15 452 11 29.3 443 m-Aramid 0.8 26 Com. Ex. 1-3 — —11 29.3 443 m-Aramid 0.8 87 Com. Ex. 1-4 — — 11 29.3 443 m-Aramid 2.0 45Com. Ex. 1-5 15 412 12 21.4 300 m-Aramid 0.8 45 Com. Ex. 1-6 15 482 1129.3 443 m-Aramid — — Com. Ex. 1-7 16 372 11 29.3 443 m-Aramid 0.8 34Com. Ex. 1-8 17 395 11 29.3 443 m-Aramid  0.013 45 Com. Ex. 1-9 181,568   11 29.3 443 p-Aramid 0.8 13 Heat-resistant Heat shrinkage porouslayer (175° C.) Coating MD TD Slidability 2 thickness Coating directiondirection (Friction SD (μm) surface (%) (%) Slidability 1 coefficient)characteristics Ex. 1-1 4 Double 22 19 Desirable 0.52 Desirable Ex. 1-24 Double 21 18 Desirable 0.49 Desirable Ex. 1-3 7 Double 11 10 Desirable0.45 Desirable Ex. 1-4 5 Single 19 18 Desirable 0.44 Desirable Ex. 1-5 7Double 14 13 Desirable 0.51 Desirable Com. Ex. 1-1 4 Double 37 23Undesirable 0.95 Undesirable Com. Ex. 1-2 4 Double 53 36 Undesirable0.75 Undesirable Com. Ex. 1-3 — Double — — — — — Com. Ex. 1-4 — Double —— — — — Com. Ex. 1-5 4 Double 15 10 Desirable 0.53 Undesirable Com. Ex.1-6 4 Double 37 20 Undesirable 0.90 Undesirable Com. Ex. 1-7 5 Single 4130 Desirable 0.64 Desirable Com. Ex. 1-8 6 Double 21 15 Undesirable 0.88Desirable Com. Ex. 1-9 7 Double 48 25 Undesirable 0.86 Undesirable

Example 1-6

A cathode paste was prepared by dissolving a lithium cobalt oxide(LiCoO₂, Nippon Chemical Industrial Co., Ltd.; cathode active material)powder, an acetylene black (Denka Black, Denki Kagaku Kogyo) powder, andpolyvinylidene fluoride (Kureha) in N-methylpyrrolidone at theproportions of 89.5 parts by weight, 4.5 parts by weight, and 6 parts byweight, respectively, in terms of a dry weight. In the cathode paste,the content of the polyvinylidene fluoride was 6 weight %. The resultingpaste was applied over an aluminum foil having a thickness of 20 μm,which was then dried and pressed to obtain a cathode.

An anode paste was prepared by dissolving a graphitized mesophase carbonmicrobead (MCMB, Osaka Gas Chemicals Co., Ltd.; anode active material)powder, acetylene black, and polyvinylidene fluoride inN-methylpyrrolidone at the proportions of 87 parts by weight, 3 parts byweight, and 10 parts by weight, respectively, in terms of a dry weight.In the anode paste, the content of the polyvinylidene fluoride was 6weight %. The resulting paste was applied over a copper foil having athickness of 18 μm, which was then dried and pressed to obtain an anode.

The cathode was cut into a 14 mm×20 mm size and tabbed. The anode wascut into a 16 mm×22 mm size and tabbed. The separator, produced inExample 1-1, was cut into a 20 mm×26 mm size. The cathode, theseparator, and the anode were laminated in this order and bondedtogether. Then, the separator was sealed in an aluminum laminated filmwith an electrolyte to obtain a non-aqueous secondary battery of thepresent invention. As the electrolyte, a 1 M solution of LiPF₆ dissolvedin a 3:7 weight-ratio mixture of ethylene carbonate and ethylmethylcarbonate was used.

Comparative Example 1-10

A battery for comparison was produced as in Example 1-6, except that apolyethylene microporous membrane having a thickness of 20 μm, a unitweight of 12.9 g/m², a Gurley number of 543 sec/100 cc (27.2 sec/100cc·μm), and a penetration strength of 556 g was used for the separator.

[Battery Oven Test]

Each battery produced in Example 1-6 and Comparative Example 1-10 wascharged to 4.2 V with 0.2 C under constant current and constant voltage.The battery was placed in an oven with a 5-kg weight, and the oven washeated to 200° C. Here, temperature-dependent changes on battery voltagewere measured. In the battery of Example 1-6, no abrupt voltage drop wasobserved until 200° C., suggesting that the internal shorting due to themeltdown of the separator was prevented. In contrast, in the battery ofComparative Example 1-10, an abrupt voltage drop occurred as a result ofinternal shoring at temperatures near 145° C. These results demonstratethat the non-aqueous secondary battery of a configuration of the presentinvention can avoid the risk associated with the internal shorting alsoin a high-temperature environment.

Example 1-7

A non-aqueous secondary battery of the present invention was produced asin Example 1-6, except that the separator produced in Example 1-3 wasused.

Example 1-8

A non-aqueous secondary battery of the present invention was produced asin Example 1-6, except that the separator produced in Example 1-4 wasused. The heat-resistant porous layer formed of Conex and alumina wasdisposed on the anode side.

Example 1-9

A non-aqueous secondary battery of the present invention was produced asin Example 1-6, except that the separator produced in Example 1-4 wasused. The heat-resistant porous layer formed of Conex and alumina wasdisposed on the cathode side.

[Trickle Charge Test]

A trickle charge test was performed using the batteries of the presentinvention produced in Examples 1-7, 1-8, and 1-9, and the comparativebattery produced in Comparative Example 1-10. The trickle charge testwas conducted over a time course of 400 hours, by continuously chargingthe battery to maintain a battery voltage of 4.3 V in a temperatureenvironment of 60° C.

In the battery of Comparative Example 1-10, a leak current started toflow after 50 hours, making it difficult to maintain the battery voltageat 4.3 V. The battery was disassembled after the test to observe theseparator. The separator had carbonized areas, appearing black, over itsentire surface.

In the batteries of the present invention produced in Examples 1-7, 1-8,and 1-9, no prominent leak current was observed in the test, and avoltage of 4.3 V was maintained. The small residual current, required tomaintain the voltage of 4.3 V, was the smallest in Example 1-7, andbecame larger in Examples 1-9 and 1-8, in this order. The battery wasdisassembled after the test to observe the separator. In the separatorof Example 1-7, hardly any discoloration or other changes were observed,whereas, in Examples 1-9 and 1-8, discoloration was observed on thesurface not coated with the heat-resistant porous layer. The separatorof Example 1-9 was dark brown in color, and the separator of Example 1-8appeared black by discoloration.

It can be seen from these results that coating the polyethylenemicroporous membrane with the porous layer of wholly aromatic polyamideand inorganic fine particles is also effective from the standpoint ofimproving durability. Specifically, the polyethylene microporousmembrane at the cathode-anode interface degrades under severeconditions, and generates a current leak. In this regard, theheat-resistant porous layer of wholly aromatic polyamide and inorganicfine particles is highly stable, and the provision of the heat-resistantporous layer prevents a current leak even under conditions where thepolyethylene microporous membrane would otherwise degrades. Note,however, that degradation does occur on the exposed surface of thepolyethylene microporous membrane in a single-coated separator. It istherefore preferable, from the stand point of durability, to form adouble-sided coating. Further, concerning durability, it is morepreferable to dispose the heat-resistant porous layer on the cathodeside in a single-coated separator.

Example 1-10

A non-aqueous secondary battery of the present invention was produced asin Example 1-6, except that the separator of Example 1-5 was used.

The non-aqueous secondary battery was subjected to the trickle chargetest as above. In this non-aqueous secondary battery, no leak currentwas observed, and the voltage was maintained at 4.3 V. However, thesmall residual current for maintaining the 4.3 V voltage was larger thanin the battery of Example 1-7. The separator, observed by disassemblingthe battery after the trickle charge test, appeared brown bydiscoloration. The result therefore shows that the para-type whollyaromatic polyamide is not deficient in terms of durability, but is notas preferable as the meta-type wholly aromatic polyamide.

Examples According to the Second Aspect of the Present Invention

Examples according to the second aspect of the present invention aredescribed below.

Measurement Methods

In the Examples according to the second aspect of the present invention,the measurements of Gurley number, thickness, heat shrinkage rate, andpenetration strength were performed as in the first aspect of thepresent invention, and no further explanation will be made regarding themeasurement methods.

[Coating Amount]

The coating amount of meta-type wholly aromatic polyamide was determinedby subtracting the unit weight of the polyethylene microporous membranefrom the unit weight of the separator produced. The unit weight wasdetermined by measuring the weight of a sample piece cut into a 33 cm(MD direction)×6 cm (TD direction) size, and converting this weight intoa weight per 1 m² area.

[Porosity]

The bulk density d1 of the meta-type wholly aromatic polyamide porouslayer was determined from the thickness of this layer and the coatingamount. The porosity ε was determined from the following equation:

ε=(1−d1/d2)×100,

where d2 is the true density of the meta-type wholly aromatic polyamide.

Reference Example 1

A polyethylene microporous membrane was used that had a unit weight of6.99 g/m², a thickness of 11 μm, a Gurley number of 322 seconds (29.3sec/100 cc·μm), heat shrinkage rates of 5.0% and 3.5% in MD directionand TD direction, respectively, at 105° C., and a penetration strengthof 443 g. The polyethylene microporous membrane had a weight averagemolecular weight of 1,270,000.

As the meta-type wholly aromatic polyamide, thepoly-m-phenyleneisophthalamide Conex® (Teijin Techno Products Limited)was used. The Conex was dissolved in a 60:40 weight-ratio mixed solventof dimethylacetoamide (DMAc) and tripropyleneglycol (TPG) so as to makethe Conex content 6 weight %. The resultant mixture was obtained as thecoating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. The polyethylene microporousmembrane was immersed in a solidifying liquid at 40° C. (water:mixedsolvent=50:50, weight ratio), followed by water washing and drying. As aresult, a Conex porous layer was formed on the both surfaces of thepolyethylene microporous membrane. Here, the clearance between the twoMeyer bars, and the size of the Meyer bars were adjusted to vary thecoating amount. The heat shrinkage rate of each sample was measured at175° C. The result of measurement is represented in the graph of FIG. 3,in which the horizontal axis represents coating amount (unit amount),and the vertical axis represents heat shrinkage rate.

Reference Example 2

Samples with varying coating amounts were produced as in ReferenceExample 1, except that the weight ratio of DMAc and TPG in the mixedsolvent was 70:30. The heat shrinkage rate of each sample was measuredat 175° C. The result of measurement is represented in the graph of FIG.3, in which the horizontal axis represents coating amount, and thevertical axis represents heat shrinkage rate.

It can be seen from FIG. 3 that the heat shrinkage rate abruptlydecreases with increase in coating amount, and substantially levels offwith the coating amounts of 2 g/m² and higher.

Example 2-1

A polyethylene microporous membrane was used that had a unit weight of6.99 g/m², a thickness of 11 μm, a Gurley number of 322 seconds (29.3sec/100 cc·μm), heat shrinkage rates of 5.0% and 3.5% in MD directionand TD direction, respectively, at 105° C., and a penetration strengthof 443 g. The polyethylene microporous membrane had a weight averagemolecular weight of 1,270,000.

As the meta-type wholly aromatic polyamide, thepoly-m-phenyleneisophthalamide Conex® (Teijin Techno Products Limited)was used. The Conex was dissolved in a 60:40 weight-ratio mixed solventof dimethylacetoamide (DMAc) and tripropyleneglycol (TPG) so as to makethe Conex content 6 weight %. The resultant mixture was obtained as thecoating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. The clearance between theMeyer bars was adjusted to 30 μm, and the both Meyer bars were of size8. The polyethylene microporous membrane was immersed in a solidifyingliquid at 40° C. (water:mixed solvent=50:50, weight ratio), followed bywater washing and drying. As a result, a Conex porous layer was formedon the both surfaces of the polyethylene microporous membrane. Table 2represents properties of the separator for a non-aqueous secondarybattery according to Example 2-1.

Example 2-2

A separator for a non-aqueous secondary battery of the present inventionwas obtained as in Example 1, except that the solidifying liquid usedfor solidification had a temperature of 60° C., and a water:mixedsolvent weight ratio of 80:20. Table 2 represents properties of theseparator for a non-aqueous secondary battery according to Example 2-2.

Comparative Example 2-1

A polyethylene microporous membrane was used that had a unit weight of6.99 g/m², a thickness of 11 μm, a Gurley number of 322 seconds (29.3sec/100 cc·μm), heat shrinkage rates of 5.0% and 3.5% in MD directionand TD direction, respectively, at 105° C., and a penetration strengthof 443 g. The polyethylene microporous membrane had a weight averagemolecular weight of 1,270,000.

As the meta-type wholly aromatic polyamide, thepoly-m-phenyleneisophthalamide Conex (Teijin Techno Products Limited)was used. The Conex was dissolved in a 70:30 weight-ratio mixed solventof dimethylacetoamide (DMAc) and tripropyleneglycol (TPG) so as to makethe Conex content 6 weight %. The resultant mixture was obtained as thecoating liquid.

Two Meyer bars were disposed opposite to each other, and an appropriateamount of the coating liquid was placed therebetween. The polyethylenemicroporous membrane was passed between the Meyer bars holding thecoating liquid, so as to apply the coating liquid on the both surfacesof the polyethylene microporous membrane. The clearance between theMeyer bars was adjusted to 30 μm, and the both Meyer bars were of size8. The polyethylene microporous membrane was immersed in a solidifyingliquid at 40° C. (water:mixed solvent=50:50, weight ratio), followed bywater washing and drying. As a result, a Conex porous layer was formedon the both surfaces of the polyethylene microporous membrane. Table 2represents properties of the separator for a non-aqueous secondarybattery according to Comparative Example 2-1.

Comparative Example 2-2

A separator for comparison was obtained as in Comparative Example 1,except that the solidifying liquid used for solidification had atemperature of 40° C., and a water:mixed solvent weight ratio of 80:20.Table 2 represents properties of the separator for a non-aqueoussecondary battery according to Comparative Example 2-2.

Comparative Example 2-3

A polyethylene microporous membrane was used that had a unit weight of7.72 g/m², a thickness of 12 μm, a Gurley number of 257 seconds (21.4sec/100 cc·μm, heat shrinkage rates of 4.9% and 2.7% in MD direction andTD direction, respectively, at 105° C., and a penetration strength of300 g. The polyethylene microporous membrane had a weight averagemolecular weight of 1,070,000.

A meta-type wholly aromatic polyamide porous layer was formed on thepolyethylene microporous membrane as in Example 2-1 to obtain acomparative separator. Table 2 represents properties of the separatorfor a non-aqueous secondary battery according to Comparative Example2-3.

Comparative. Example 2-4

A polyethylene microporous membrane was used that had a unit weight of7.72 g/m², a thickness of 12 μm, a Gurley number of 257 seconds (21.4sec/100 cc·μm), heat shrinkage rates of 4.9% and 2.7% in MD directionand TD direction, respectively, at 105° C., and a penetration strengthof 300 g. The polyethylene microporous membrane had a weight averagemolecular weight of 1,070,000.

A meta-type wholly aromatic polyamide porous layer was formed as inComparative Example 2-1, except for using the polyethylene microporousmembrane above, to obtain a comparative separator. Table 2 representsproperties of the separator for a non-aqueous secondary batteryaccording to Comparative Example 2-4.

TABLE 2 Polyethylene microporous membrane Heat-resistant porous layerGurley Coating Coating Thickness number (sec/ amount thickness Porosity(μm) 100 cc · μm) (g/m²) (μm) (%) Ex. 2-1 11 29.3 2.25 4 58 Ex. 2-2 −1129.3 2.07 3 −49 Com. Ex. 2-1 11 29.3 2.22 5 67 Com. Ex. 2-2 11 29.3 2.074 61 Com. Ex. 2-3 12 21.4 2.40 6 70 Com. Ex. 2-4 12 21.4 2.11 6 73

Example 2-3

A cathode paste was prepared by dissolving a lithium cobalt oxide(LiCoO₂, Nippon Chemical Industrial Co., Ltd.; cathode active material)powder, an acetylene black (Denka Black, Denki Kagaku Kogyo) powder, andpolyvinylidene fluoride (Kureha) in N-methylpyrrolidone at theproportions of 89.5 parts by weight, 4.5 parts by weight, and 6 parts byweight, respectively, in terms of a dry weight. In the cathode paste,the content of the polyvinylidene fluoride was 6 weight %. The resultingpaste was applied over an aluminum foil having a thickness of 20 μm,which was then dried and pressed to obtain a cathode.

An anode paste was prepared by dissolving a graphitized mesophase carbonmicrobead (MCMB, Osaka Gas Chemicals Co., Ltd.; anode active material)powder, acetylene black, and polyvinylidene fluoride inN-methylpyrrolidone at the proportions of 87 parts by weight, 3 parts byweight, and 10 parts by weight, respectively, in terms of a dry weight.In the anode paste, the content of the polyvinylidene fluoride was 6weight %. The resulting paste was applied over a copper foil having athickness of 18 μm, which was then dried and pressed to obtain an anode.

The cathode was cut into a 14 mm×20 mm size and tabbed. The anode wascut into a 16 mm×22 mm size and tabbed. The separator, produced inExample 2-1, was cut into a 20 mm×26 mm size. The cathode, theseparator, and the anode were laminated in this order and bondedtogether. Then, the separator was sealed in an aluminum laminated filmwith an electrolyte to obtain a non-aqueous secondary battery of thepresent invention. As the electrolyte, a 1 M solution of LiPF₆ dissolvedin a 3:7 weight-ratio mixture of ethylene carbonate and ethylmethylcarbonate was used.

Comparative Example 2-5

A battery for comparison was produced as in Example 2-3, except that apolyethylene microporous membrane having a thickness of 20 μm, a unitweight of 12.9 g/m², a Gurley number of 543 sec/100 cc (27.2 sec/100cc·μm), and a penetration strength of 556 g was used for the separator.

[Battery Oven Test]

Each battery produced in Example 2-3 and Comparative Example 2-5 wascharged to 4.2 V with 0.2 C under constant current and constant voltage.The battery was placed in an oven with a 5-kg weight, and the oven washeated to 200° C. Here, temperature-dependent changes on battery voltagewere measured. In the battery of Example 2-3, no abrupt voltage drop wasobserved until 200° C., suggesting that the internal shorting due to themeltdown of the separator was prevented. In contrast, in the battery ofComparative Example 2-5, an abrupt voltage drop occurred as a result ofinternal shoring at temperatures near 145° C. These results demonstratethat the non-aqueous secondary battery of the configuration of thepresent invention can avoid the risk associated with the internalshorting also in a high-temperature environment.

[Evaluation of Shutdown Characteristics]

The shutdown characteristics were evaluated in regard to the separatorsof Examples 2-1 and 2-2, and Comparative Examples 2-1 through 2-4, as inthe Examples according to the foregoing first aspect of the presentinvention.

The results are represented in FIG. 4. By comparing the Examples and theComparative Examples, it can be seen that the separators coated with theheat-resistant porous layer can exhibit a desirable shutdown functiononly if they have a configuration of the present invention.

INDUSTRIAL APPLICABILITY

A separator of the present invention effectively ensures safety ofnon-aqueous secondary batteries. The separator is therefore suitable foruse in high-energy-density, high-capacity, or high-output non-aqueoussecondary batteries.

1. A separator for a non-aqueous secondary battery, comprising: amicroporous membrane of primarily polyethylene; and a heat-resistantporous layer of primarily at least one kind of heat-resistant polymerselected from the group consisting of a wholly aromatic polyamide, apolyimide, a polyamide-imide, a polysulfone, and a polyether sulfone,the heat-resistant porous layer being formed on at least one surface ofthe microporous membrane, the separator characterized in that: (1) themicroporous membrane has a Gurley number of 25 to 35 sec/100 cc·μm perunit thickness; (2) the heat-resistant porous layer contains inorganicfine particles having an average particle diameter of 0.1 to 1.0 μm; (3)the inorganic fine particles are 40% to 80% in volume with respect to atotal volume of the heat-resistant polymer and the inorganic fineparticles; and (4) the heat-resistant porous layer has a total thicknessof 3 to 12 μm when formed on both surfaces of the microporous membrane,and a thickness of 3 to 12 μm when formed on only one surface of themicroporous membrane.
 2. The separator according to claim 1, wherein theheat-resistant porous layer is formed on the both surfaces of themicroporous membrane.
 3. The separator according to claim 1, wherein theheat-resistant polymer is a wholly aromatic polyamide.
 4. The separatoraccording to claim 3, wherein the wholly aromatic polyamide is ameta-type wholly aromatic polyamide.
 5. The separator according to claim1, wherein the heat-resistant porous layer is formed on the bothsurfaces of the microporous membrane, wherein the microporous membraneis formed of polyethylene, and wherein the heat-resistant polymer is ameta-type wholly aromatic polyamide.
 6. The separator according to claim4, wherein the meta-type wholly aromatic polyamide is apoly-m-phenyleneisophthalamide.
 7. The separator according to claim 1,wherein the inorganic fine particles are made of alumina, and whereinthe inorganic fine particles are 65% to 90% in weight with respect to atotal weight of the heat-resistant polymer and the inorganic fineparticles.
 8. The separator according to claim 7, wherein the inorganicfine particles are made of α-alumina.
 9. The separator according toclaim 1, wherein the microporous membrane has a penetration strength of250 g or more.
 10. The separator according to claim 1, wherein themicroporous membrane has a thickness of 7 to 16 μm, and wherein theseparator has a thickness of 20 μm or less as a whole.
 11. A producingprocess of a separator for a non-aqueous secondary battery, theseparator including a microporous membrane of primarily polyethylene,and a heat-resistant porous layer of a primarily wholly aromaticpolyamide formed on at least one surface of the microporous membrane,the process characterized by comprising: (1) applying a coating liquidto at least one surface of the microporous membrane, the coating liquidincluding a wholly aromatic polyamide, inorganic fine particles, asolvent for dissolving the wholly aromatic polyamide, and a solvent thatserves as a poor solvent for the wholly aromatic polyamide; (2)solidifying the coating liquid by immersing the microporous membrane ina mixture of the solvent for dissolving the wholly aromatic polyamide,and the solvent that serves as a poor solvent for the wholly aromaticpolyamide, after applying the coating liquid to the microporousmembrane; (3) performing water washing to remove the solvent mixture;and (4) performing drying to remove the water.
 12. The process accordingto claim 11, wherein the coating liquid is a slurry in which the whollyaromatic polyamide is dissolved, and in which the inorganic fineparticles are dispersed.
 13. The process according to claim 12, whereinthe wholly aromatic polyamide is a meta-type wholly aromatic polyamide.14. A non-aqueous secondary battery using a separator of claim
 1. 15. Aseparator for a non-aqueous secondary battery, comprising: a microporousmembrane of primarily polyethylene; and a heat-resistant porous layer ofprimarily at least one kind of heat-resistant polymer selected from thegroup consisting of a wholly aromatic polyamide, a polyimide, apolyamide-imide, a polysulfone, and a polyether sulfone, theheat-resistant porous layer being formed on at least one surface of themicroporous membrane, the separator characterized in that: (1) themicroporous membrane has a Gurley number of 25 to 35 sec/100 cc·μm perunit thickness; (2) the microporous membrane has a thickness of 7 to 16μm; (3) the heat-resistant polymer is coated in an amount of 2 to 3g/m²; (4) the heat-resistant porous layer has a total thickness of 3 to7 μm when formed on both surfaces of the microporous membrane, and athickness of 3 to 7 μm when formed on only one surface of themicroporous membrane; and (5) the heat-resistant porous layer has aporosity of 40% to 60%.
 16. The separator according to claim 15, whereinthe microporous membrane has a heat shrinkage rate at 105° C. of 10% orless both in a MD direction and a TD direction.
 17. The separatoraccording to claim 15, wherein the microporous membrane has apenetration strength of 250 g or more.
 18. The separator according toclaim 15, wherein the heat-resistant porous layer is formed on the bothsurfaces of the microporous membrane, wherein the microporous membraneis formed of polyethylene, and wherein the heat-resistant polymer is ameta-type wholly aromatic polyamide.
 19. The separator according toclaim 18, wherein the meta-type wholly aromatic polyamide is apoly-m-phenyleneisophthalamide.
 20. A non-aqueous secondary batteryusing a separator of claim 15.